The present invention relates to a method for synchronization of a light grid and to a light grid based on this method.
The invention generally relates to the field of machine safety in the sense of protecting persons and/or material values against dangers which result from automatically operating systems or machines, for example automatically operated robots. As a typical protective measure the danger zone around f such a system is secured so that entering the danger zone is either prevented or leads to the dangerous operating mode being halted, switched off, or otherwise defused. To secure such a danger zone, frequently mechanical safety fences or safety doors are used. However, in many cases it is necessary to access the protected danger zone, whether because an operator regularly needs to access the machine and/or because material must be transported into or out of the danger zone. Optoelectronic protection devices are frequently used for such instances, in particular light grids or light curtains.
Light grids comprise a transmitting unit with a plurality of individual transmitters which transmit the transmission beams to a receiving unit at a distance. The receiving unit has a plurality of receivers, in which a transmitter of the transmitting unit is respectively assigned a receiver in the receiving unit which together form a transmitter/receiver pair. The transmission beams usually run in parallel to each other between the transmitters and the receivers and thereby span a protective field which represents the region to be monitored. If one of the transmission beams is interrupted, for instance, by a body part of a person, said interruption can be detected with the aid of the receivers, and the system to be protected can be halted or switched off.
Light grids are typically used in a scanning mode, that is to say the individual transmitter/receiver pairs are cyclically activated one after another in a scanning direction so that only one transmitter/receiver pair is active at a point in time. A precondition for the scanning mode is that the transmitter control unit and the receiver control unit of the transmitting and receiving unit can be synchronized in terms of time with one another so that an associated receiver can be activated synchronously to a transmitter. However, since the transmitting and receiving units are at a distance from one another, there is generally no electric coupling, for example by cable or radio, between the two units. The synchronization is performed in said instances in an optical fashion via the transmission beams.
DE 195 10 304 C1 shows such a light grid in which the synchronization is performed in an optical fashion, wherein an individual transmitter/receiver pair being used for the synchronization of the remaining transmitter/receiver pairs. The transmitter of said transmitter/receiver pair emits a synchronization beam which is different from the other transmission beams. The associated receiver is constantly activated. Once the receiver receives the synchronization beam, the cyclic activation of the downstream receivers begins from said receiver. The light grid can therefore be newly synchronized after each pass through the scanning cycle.
In many operations, the protective field of a light grid extends further than the region actually to be monitored. For example, fixed machine parts may be located within the protective field yet said parts should not trigger the safety function. Likewise, other none safety-critical objects may be inserted into the machine such as workpieces, which likewise should not lead to a shutdown off of the system. In order to adapt the protective field to the safety-critical regions, subregions of the protective field are frequently defined as so called blanking or muting regions in which detection of an object should not trigger the safety function. In other words, individual transmitter and receiver pairs are selectively eliminated in subregions, either permanently (blanking) or only for short periods (muting).
In order to be able to use a light grid flexibly, and since it is generally not known before the installation of the light grid which regions are to be defined as blanking regions, it is desirable for the light grid to be designed so that all the transmitter/receiver pairs are suitable for blanking. However, at the same time synchronization in an optical fashion should be feasible. A light grid according to DE 195 10 304 C1, a predefined transmitter/receiver pair is used for the synchronization. If said transmitter/receiver pair is eliminated because of blanking, synchronization is not possible.
DE 10 2005 047 776 B4 describes a method in which optical synchronization is not performed based on a predefined transmitter/receiver pair, but can be carried out with any transmitter/receiver pair. For this purpose, a unique identifier is assigned to each individual transmitter/receiver pair, said identifier enabling a unique association of the transmitter/receiver pair from the totality of the transmitter/receiver pairs. If a transmission beam with an associated identifier is detected by a receiver, synchronization can be performed on the basis of the unique assignment. However, said unique assignment presupposes that the transmission beams respectively differ from one another, and therefore requires a plurality of different identifiers for the individual transmitter/receiver pairs. However, this requires either that transmitter/receiver pairs are of different design, or instead that the control units in the transmitting and receiving unit are of a correspondingly complex design. Both variants increase manufacturing costs of the light grid.
DE 10 2007 059 565 B4 describes a further method, in which the transmitter/receiver pair used for the synchronization is not predefined. In this case, the synchronization is performed with the aid of a coded transmit beam, but uses the fact that there is no need for an absolute assignment of the transmit beam to a transmitter/receiver pair, but a relative assignment is already sufficient for the synchronization, that is to say on detection of the coded transmit beam at a receiver it is possible to start cyclic activation with the downstream receiver on the basis of the sequential control. In order for the coded transmit beam to be applied to different transmitter/receiver pairs, the control unit in the transmitting unit has an algorithm which emits the coded transmit beam via different transmitter/receiver pairs in a temporally varying fashion. Since the coded transmit beam continues to be applied to different transmitter/receiver pairs, blanking individual beams does not prevent synchronization taking place. In the most unfavorable instance, only several cycles are required for the synchronization. In contrast to the aforementioned methods, a single identifier differing from the others is required in these methods. However, it is disadvantageous that only a relative assignment of the coded transmit beam is possible to a transmitter/receiving pair.